1. Field of the Invention
This invention relates to a nuclear reactor wherein liquid metal is used as primary and secondary coolants, and more particularly to a nuclear reactor which comprises a nuclear reactor vessel and a core built in the reactor vessel to be cooled by the primary coolant.
2. Background of the Prior Art
A coolant for the above-mentioned type of nuclear reactor is generally formed of, for example, liquid sodium or a mixture of liquid sodium and potassium. With such type of nuclear reactor, a primary coolant passing through a core assumes strong radioactivity. Since such radioactive coolant, if directly supplied to a steam generator, gives rise to serious hazard to the human body, the primary coolant is allowed to flow through a core and intermediate heat exchanger by means of a circulation pump. After heat exchange is carried out between the primary and secondary coolants in said intermediate heat exchanger, the secondary coolant is conducted to a steam generator to produce high pressure steam.
The conventional nuclear reactor of the above-mentioned type can be broadly classified into two types: the loop type and pool type. With the loop type, a core is set in a nuclear reactor vessel, and a primary coolant circulation pump and intermediate heat exchanger are disposed outside of the reactor vessel. All the above-mentioned members are connected together by proper piping to constitute a closed loop allowing for the passage of the primary coolant. With the closed loop, a hot primary coolant delivered from a core is carried to a circulation pump, which discharges the primary coolant in a pressurized state. Heat exchange takes place between the discharged primary coolant and secondary coolant in the intermediate heat exchanger. The primary coolant which has been drawn off from the intermediate heat exchanger and whose temperature has now decreased is sent back to the core of the reactor vessel. However, the loop type nuclear reactor has the drawbacks that a primary coolant pipe constructed outside of the reactor vessel has a complicated arrangement; not only said piping but also the reactor itself is rendered bulky, resulting in the enlargement of a nuclear reactor building; and the piping of the hot primary coolant is subject to so great a thermal stress that a complicated design must be developed to cope with said prominent thermal stress.
With the pool type, the main vessel of the above-mentioned loop type reactor comprises a core, primary coolant circulation pump, and intermediate heat exchanger. Partition walls are provided between these members, causing the primary coolant to run through the spaces defined between the partition walls and said members. The highly pressurized primary coolant discharged from the circulation pump is supplied to the core. The primary coolant rendered extremely hot while passing through the core is conducted to the intermediate heat exchange. After heat-exchanged with the secondary coolant, the primary coolant runs through the vessel back to the circulation pump.
With the pool type reactor, the primary coolant is circulated within the main vessel, making it unnecessary to provide a primary coolant pipe outside of the vessel, and indeed reducing the size of the nuclear reactor as a whole. However, the above-mentioned pool type reactor has the drawbacks that the main vessel containing the primary coolant circulation pump and the intermediate heat exchanger becomes too large to be carried as an undivided unit, and has to be manufactured in parts and put together at a site of installation, presenting difficulties in the manufacture, transport and assembly of the vessel and consequently increasing the cost of a nuclear reactor as a whole; the main vessel containing the primary coolant circulation pump and intermediate heat exchanger has its inner space substantially filled, leaving too little room for the maintenance and repair of the equipment received in the vessel; the primary coolant circulation pump has to be set between the main vessel which indicates different diametric expansions and its roof; and consequently the primary coolant circulation pump presents considerable difficulties and complications in construction and installation in order to absorb the effect of the difference in the above-mentioned diametric thermal expansion which might otherwise act on said circulation pump.